(a) Technical Field
The present invention relates to a gas diffusion layer and a fuel cell stack comprising the same. More particularly, it relates to a local hydrophilic gas diffusion layer configured to enhance the water removal performance of a fuel cell, and a fuel cell stack comprising the same.
(b) Background Art
A fuel cell is an electrical generation system that does not convert chemical energy of fuel into heat by combustion, but rather electrochemically converts the chemical energy directly into electrical energy in a fuel cell stack. Fuel cells can be applied to the electric power supply of small-sized electrical and electronic devices, for example portable devices, as well as industrial and household appliances and vehicles.
One of the most widely used fuel cells for a vehicle is a proton exchange membrane fuel cell or a polymer electrolyte membrane fuel cell (PEMFC), which includes a fuel cell stack having a membrane electrode assembly (MEA), a gas diffusion layer (GDL), a gasket, a sealing member, and a bipolar plate (separator). Generally, the MEA includes a polymer electrolyte membrane, through which hydrogen ions are transported and an electrode/catalyst layer, in which an electrochemical reaction takes place, is disposed on each of both sides of the polymer electrolyte membrane. The GDL functions to uniformly diffuse reactant gases and transmit generated electricity. The gasket functions to provide an appropriate airtightness to reactant gases and coolant. The sealing member functions to provide an appropriate bonding pressure. The bipolar plate functions to support the MEA and GDL, collect and transmit generated electricity, transmit reactant gases, transmit and remove reaction products, and transmit coolant to remove reaction heat, etc.
The fuel cell stack is composed of a plurality of unit cells, each of the unit cells including an anode, a cathode, and an electrolyte (electrolyte membrane). Hydrogen, as fuel, is supplied to the anode (“fuel electrode”, “hydrogen electrode” or “oxidation electrode”) and oxygen as oxidant is supplied to the cathode (“air electrode”, “oxygen electrode” or “reduction electrode”).
The hydrogen supplied to the anode is dissociated into hydrogen ions (protons, H+) and electrons (e) by a catalyst disposed in the electrode/catalyst layer. The hydrogen ions are transmitted to the cathode through the electrolyte membrane, (a cation exchange membrane), and the electrons are transmitted to the cathode through the GDL and the bipolar plate.
At the cathode, the hydrogen ions supplied through the (polymer) electrolyte membrane and the electrons transmitted through the bipolar plate react with the oxygen in the air supplied to the cathode to produce water.
Migration of the hydrogen ions causes electrons to flow through an external conducting wire, which generates electricity and heat.
The electrode reactions in the fuel cell can be represented by the following formulas:
Reaction at the anode: 2H2→4H++4e−
Reaction at the cathode: O2+4H++4e−→+2H2O
Overall reaction: 2H2+O2→2H2O+electrical energy+heat energy
As shown in the above formulas, water is produced from the reaction occurring in the fuel cell. It is known that the water content in the fuel cell stack is directly related to the humidity of the electrolyte membrane, the flow of hydrogen as the fuel, the flow of air as the oxidant, and the durability of the electrode catalyst. Therefore, the management of produced water is a very important technique that ultimately determines the performance of the fuel cell.
U.S. Pat. No. 6,967,039 discloses a diffusion media and a process for its fabrication to address issues related to water management in electrochemical cells. The diffusion media includes a mesoporous layer (MPL) formed by providing a coating having a hydrophobic component, a hydrophilic component, and a pore forming agent on a substrate formed of a carbon material having excellent electrical conductivity.
Moreover, U.S. Pat. No. 7,332,240 discloses a diffusion media including a mesoporous layer which is divided into a high water region and a low water region so as to enhance water transfer properties of at least one of first and second diffusion layer substrates in the high water region and diminish water transfer properties of at least one of first and second diffusion layer substrates in the low water region, thus ensuring water preservation.
However, the diffusion media, including the mesoporous layer, simultaneously have water repellent properties and water retention properties due to the nature of the structure of the mesoporous layer, and thus it is practically difficult to divide the regions according to the amount of water. Especially, in the case of a serpentine channel, it is quite difficult to divide the regions. In detail, the region where water is accumulated is changed according to the amount of current, the operating temperature, and the gas moisture content in the fuel cell stack, and thus the amount of water produced and the drainage capacity do not accord with each other. Therefore, it is difficult to apply the division of the regions to the serpentine channel system.
Meanwhile, U.S. Pat. No. 7,250,189 discloses an electroconductive porous substrate, such as carbon fiber paper with an electroconductive polymer deposited on the carbon fibers of the paper, used as a wicking material or diffusion medium in a fuel cell.
However, to remove water through the wicking mechanism, the entire gas diffusion layer should be hydrophilic. Although it is advantageous to allow water to flow from the electrode to the gas diffusion layer, it is disadvantageous to allow water to be released to a gas channel, and thus it is difficult to achieve the desired object of removing the produced water.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.